Unlikely — Zone Mismatch
key lime (zones 10-11) does not overlap with Washington's zone range (4a-9b). Growing outdoors is unlikely without significant protection.
One zone short — what would close it
Key Lime is hardy to zone 10, and the warmest ground in Washington is zone 9 — 1 zone short. Planted in open ground it will not survive a Washington winter. To grow it here, grow key lime in a container and move it under cover for the coldest weeks, or give it a greenhouse.
Season extension and overwintering under cover both work at this margin. How to buy the weeks →
Zone math from 10-11 (plant) against 5-9 (Washington). Lift modeled by crop_engine Sprint 20B effective_zone_modifier (+2 zones) via api/fast_sections.py Unlock Engine v2.
Your yard isn't the whole zone.
Washington spans zones 4a-9b, but your yard sits in exactly one — and slope, tree cover, and cold-air pockets nudge it further. Enter your address and we'll score key lime against your parcel's actual hardiness, soil, and sun.
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Zone Comparison
Key Lime Needs
- USDA Zones: 10-11
- Frost-Free Days: 140+
- Sun: Full Sun
- Mature size: 12 ft tall
- Picking window: ~28 days of harvest
- Drought tolerance: Low
Washington Has
- USDA Zones: 4a-9b
- Last Hard Freeze (28°F): Mar 16 - Apr 15
- First Hard Freeze (28°F): Oct 24 - Nov 18
- Annual Rainfall: 6-90 inches
- Common Soils: Volcanic ash, Silt loam (Palouse), Sandy loam
Plant Zone Range (zones 10-11)
Preferred Soil pH
Plant data: USDA PLANTS Database / Growable Ground plant catalog. State data: USDA ARS PHZM 2023, NOAA Climate Normals, NRCS SSURGO.
When to Plant Key Lime in Washington
The frost window
Across Washington, the last spring hard freeze (28°F) clears between Mar 16 and Apr 15, and the first fall hard freeze lands between Oct 24 and Nov 18 (NOAA Regional Climate Centers ACIS, 1991-2020). Counting from the latest last freeze to the earliest first freeze, that's a 192-day window you can count on — up to 247 days on a mild site in a kind year. That marks the hard freeze, not the last light frost, so tender transplants usually wait another 2-3 weeks.
Winter standing
Washington runs as cold as USDA zone 4, and no sourced winter-hardiness floor is on record for key lime, so this page does not state whether it overwinters here — and gives no fall establishment date on the strength of a guess. Whether key lime is a permanent planting or a replant-every-spring crop for you turns on your own zone within Washington; check its hardiness range against that before treating it as long-lived ground.
Timing tuned to sub-state frost dates — Whatcom County, not the statewide average.
Frost window: NOAA Climate Normals 1991-2020. Plant timing fields: USDA PLANTS Database. Your site's own frost dates can run earlier or later than the state range — a parcel report pins them down.
Growing Season Fit
The zone check says key lime does not survive a Washington winter in the ground. The season figures below measure a different question — whether it could finish a season here at all — and they only bear on Washington under protection.
Frost-free days
Key Lime wants 140+ frost-free days; a typical Washington site sees ~208 (NOAA Regional Climate Centers ACIS, 1991-2020). That leaves comfortable headroom for succession planting.
Growing degree days
Key Lime needs ~4000 GDD (base 50°F) to ripen. The state median runs ~1740 GDD (from PRISM 1991-2020 climate normals), so Washington's typical season runs short on heat — pick a south-facing site and consider season extension.
Chill hours
Key Lime has no winter-chill requirement — its listed minimum is 0 hours (USDA PLANTS Database). Washington typically banks ~2500 chill hours per winter (from PRISM 1991-2020 climate normals), so chill is not a constraint on this plant here — and not evidence in its favour either. Winter survival and season length are what decide it.
GDD and chill-hour figures are computed from PRISM 1991-2020 monthly climate normals; frost-free days from NOAA Regional Climate Centers ACIS, 1991-2020. Each is the state median across its counties (P25/P75 span the within-state spread). Hardiness-zone range is USDA PHZM 2023.
Soil + Drainage Fit
Key Lime likes near-neutral soil (pH 5.5-8.3). That's the common-ground band across Washington's volcanic ash and silt loam (palouse) — a soil test confirms it for your site. It will grow across that whole span, but it does best between pH 6 and 7 — worth knowing before you decide whether a Washington soil test result is a problem or just a number. Drainage matters: this plant wants well (dry spells). If your Washington site is heavier clay or sits in a low spot, raised beds or amendment with compost solve it.
Your land, not the state average
Washington's soils run mostly silt loam, but SSURGO maps the series, texture, and drainage under your exact parcel. Key Lime needs about 59 inches of workable depth, so it is that horizon — not the surface — you are testing for. That map unit, not the state average, decides how key lime performs.
Check your parcel → Source: USDA NRCS SSURGO.
Plant pH and drainage requirements from USDA PLANTS Database. Washington soil profile from USDA NRCS SSURGO. Site-specific verification: a 30-minute soil test from your local Extension lab.
Key Lime in Washington — Quick Answer
- Verdict: Unlikely — Zone Mismatch
- Plant Zones: 10-11 (USDA PLANTS Database)
- State Zones: 4a-9b (USDA ARS PHZM 2023)
- Growing Season: 28°F hard freezes about Mar 16 - Apr 15 to Oct 24 - Nov 18 (NOAA Regional Climate Centers ACIS, 1991-2020)
What Else to Consider for Key Lime in Washington
Zone compatibility tells you about winter cold survival — but Washington growers also need to think about:
Extreme rain divide: 90+ inches west, 6 inches east of Cascades
East side requires irrigation — no rain from June through September
Slug and root rot pressure on the wet west side
Short seasons at elevation in the Cascades and northeast corners
Growing key lime here specifically
Key Lime needs pH 5.5–8.3; Washington's dominant silt loam soils may or may not deliver that, so your parcel's SSURGO map unit is the real test.
Start with a soil test on your own ground and adjust pH and texture to fit key lime's 5.5–8.3 range. How to handle it →
What to actually do about these — and the constraints that apply to every Washington grower, not just key lime — is covered on the Washington growing guide.
Timing shifts within Washington
Washington isn't one climate. In Whatcom County, the last hard freeze (28°F) holds until about May 29 — roughly 54 days later than the recorded state median — so plant key lime to your county's window, not the statewide date.
Key Lime needs about 140 days of frost-free season to complete its cycle. 38 of Washington’s 39 counties give it that outright. The tightest that does is Chelan County at 158 frost-free days — 18 to spare. Below that line you are transplanting or extending the season, not direct-sowing.
Across Washington’s 39 counties the frost-free season runs from 131 days in Whatcom County to 300 days in San Juan County — a spread of 169 days, which is why a single statewide planting date misleads. The median Washington county sees 208 frost-free days, leaving key lime about 68 days of slack on a typical parcel — room for a second sowing where the season runs long.
County last-freeze dates and frost-free day counts: NOAA Regional Climate Centers ACIS (1991-2020), county 28°F freeze (earlier than the folk 32°F "last frost"). Key Lime's 140-day requirement: USDA PLANTS / Growable Ground plant database (minimum frost-free days). A parcel report resolves your address's own frost dates.
Pollinator + Wildlife Value
Key Lime draws honeybees (high value, Growable Ground Plant Database). Planting it near vegetable beds can lift fruit set on neighboring crops. Outdoors, key lime can only work that bloom inside Washington's frost window — roughly 208 days, opening after the last 28°F freeze (Mar 16 - Apr 15) and closing at the first (Oct 24 - Nov 18), per NOAA Regional Climate Centers ACIS, 1991-2020.
Good to Know Before You Plant Key Lime
Key Lime is listed as toxic to dogs, cats (leaves, stems, peel) at a mild level (ASPCA). Most listed plants only cause brief upset — a raised bed or a fenced corner usually keeps curious pets clear.
Washington Cooperative Extension
For Washington-specific cultivar recommendations, planting calendars, and pest pressure for key lime, the canonical source is WSU Extension. Their fact sheets carry the local trial data we can't generalize across 50 states.
Is Key Lime native to Washington?
No — the USDA PLANTS Database lists Key Lime as introduced rather than native in the Lower 48, so it is not part of Washington's native flora. It grows here as a garden plant; pairing it with a few Washington natives keeps local pollinators fed too.
Looking for plants that belong here? The Washington growing guide lists USDA-documented natives for the state.
Native-range data: USDA PLANTS Database state-distribution records, accessed 2026-07-01.
Common Questions About Growing Key Lime in Washington
When can I plant Key Lime in Washington?
Washington's last spring hard freeze (28°F) clears between Mar 16 and Apr 15, and the first fall hard freeze lands between Oct 24 and Nov 18 (NOAA Regional Climate Centers ACIS, 1991-2020). No sourced winter-hardiness floor is on record for Key Lime, so this page does not state whether it overwinters in Washington and publishes no fall establishment date for it — check its hardiness range against your own zone before treating it as a permanent planting.
What hardiness zone is Key Lime grown in across Washington?
Washington spans USDA hardiness zones 4a-9b (USDA ARS PHZM 2023). Key Lime carries a range of zones 10-11, so the overlap zones are where outdoor growing is most reliable.
How many frost-free days does a typical Washington site have?
A typical Washington site sees ~208 frost-free days per year (derived from NOAA Regional Climate Centers ACIS, 1991-2020). Key Lime needs 140+ frost-free days, so check whether your local microclimate runs above or below the state average before settling on a planting date. In cooler counties like Whatcom, the freeze-free season runs shorter than the state average, so verify your own county's window.
Is Key Lime native to Washington?
No — the USDA PLANTS Database lists Key Lime as introduced rather than native in the Lower 48, so it is not part of Washington's native flora. It grows here as a garden plant; pairing it with a few Washington natives keeps local pollinators fed too.
How should I amend the soil for Key Lime in Washington?
Key Lime prefers pH 5.5-8.3 and well (dry spells) drainage (FAO ECOCROP). That sits in the common-ground band across Washington soils — a 30-minute soil test from a local Extension lab confirms it for your specific site.
Will Key Lime actually grow on my specific land in Washington?
State-level zone + climate data is a sketch. A Growable Ground parcel report scores key lime against your address's exact soil pH, drainage, sun, and frost-date data drawn from USDA SSURGO, NOAA, and PRISM — not state averages.
Check your specific parcel in Washington
State-level data is a sketch. Your Growable Ground report scores key lime against your parcel's exact soil, sun, drainage, and frost data — not zone averages.
Three things about your lot that Washington key lime averages can't show:
We read public map data around Washington — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Washington key lime average
Explore More
Growing guides for key lime
Analysis by the Growable Ground research team, grounded in USDA PLANTS, USDA NRCS SSURGO, NOAA Climate Normals (1991-2020), and named Cooperative Extension sources. How we know →
Cite this dataset
Kirchick, M. (2026). Growable Ground Plant Database [Data set]. Zenodo. https://doi.org/10.5281/zenodo.21435643
Licensed CC BY-NC 4.0. DOI 10.5281/zenodo.21435643
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